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In a used gun, I take it apart. Strip the slide off and inspect for rust, pitting and dirt. Get a flashlight and look at the barrel. Do the grooves look just clean and sharp or are they just barely there? Is the barrel dirty?
Now take the grip off the frame. This will let you look at more of the mechanism and again, look of dirt, rust and pitting.
Re-assemble it. How does the frame fit? Does it move when you shake the pistol? There should be a little slop in the slide, but not too much.